Neural stimulation system providing auto adjustment of stimulus output as a function of sensed pressure changes

ABSTRACT

A neural stimulation system automatically corrects or adjusts the stimulus magnitude in order to maintain a comfortable and effective stimulation therapy. Auto correction of the stimulus magnitude is linked to the measurement of pressure in the vicinity of the electrode-tissue interface. Because the pressure near the electrode-tissue interface can provide a measure of the electrode contacts&#39; proximity to the neural tissue, and hence quantity of electrical energy delivered to the neural tissue, a change in the measured pressure or pressure morphology indicates that the stimulation energy may need to be adjusted. Hence, changes in pressure provide a feedback mechanism that permit the system to effectively auto correct the stimulus amplitude in order to maintain a desired therapeutic effect.

This application claims the benefit of U.S. Application Ser. No. 60/357,009, filed Feb. 12, 2002, which application is incorporated herein by reference. This application is related to copending U.S. patent application Ser. No. 10/364,437, entitled “Neural Stimulation System Providing Auto Adjustment of Stimulus Output as a Function of Sensed Coupling Efficiency,” and copending U.S. patent application Ser. No. 10/364,436, entitled “Neural Stimulation System Providing Auto Adjustment of Stimulus Output as a Function of Sensed Impedance,” both filed Feb. 11, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to neural stimulation systems and, more particularly, to an output control system that automatically maintains the output of the stimulation system at a comfortable and efficacious level.

The present invention may be used in various stimulation therapies in which a neurostimulator is used to stimulate neural tissue. One example where the present invention may be employed is with stimulation of peripheral nerves, e.g., the nerves in the arms, legs, fingers, which nerves are distant from the spinal cord. The present invention may also be used in stimulation of spinal cord nerves.

Spinal cord stimulation (SCS) systems, treat chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array placed epidurally near a patient's spine. SCS is a well-accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an Implantable Pulse Generator (IPG) coupled to an array of electrodes at or near the distal end of an electrode lead. An electrode lead extension may also be used, if needed. The IPG generates electrical pulses that are delivered to neural tissue, e.g., the dorsal column fibers within the spinal cord, through the electrodes of the electrode array. In an SCS system, for example, the electrodes are implanted proximal to the dura mater of the spinal cord. Individual electrode contacts (the “electrodes”) may be arranged in a desired pattern and spacing in order to create an electrode array. Individual wires, or electrode leads, connect with each electrode in the array. The electrode leads exit the spinal cord and attach to the IPG, either directly, or through one or more electrode lead extensions. The electrode lead extension, in turn, when used, is typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.

The electrical pulses generated by the SCS stimulation system, or other neural system, are also referred to as “stimulation pulses”. In an SCS system, the stimulation pulses typically have the effect of producing a tingling sensation, also known as a paresthesia. The paresthesia helps block the chronic pain felt by the patient. The amplitude or magnitude of the stimulation pulses affects the intensity of the paresthesia felt by the patient. In general, it is desirable to have the amplitude of stimulation comfortably set to a level which produces paresthesia to block pain but not above a level that may actually result in pain apart from the native pain. Moreover, the stimulus amplitude should be set to a stimulus level lower than that which can recruit reflex motor nerves that can cause involuntary muscle contractions.

SCS and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467, teaches an electrode implant for neuro-stimulation of the spinal cord. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted, and which is controlled by an external controller.

Representative techniques known in the art for providing for the automatic adjustment of stimulation parameters of an implantable stimulator are disclosed, e.g., in U.S. Pat. Nos. 5,895,416; 5,735,887; and 4,735,204.

Patients having an SCS system have heretofore had to manually adjust the amplitude of the stimulation pulses produced by their SCS system in order to maintain the paresthesia at a comfortable level. This is necessary for a number of reasons. For example, postural changes, lead array movement (acute and/or chronic), and scar tissue maturation, all affect the intensity of the paresthesia felt by the patient. Because of these changes, i.e., because of postural changes, lead array movement, and scar tissue maturation, as well as other changes that may occur in the patient, the paresthesia can be lost, or can be converted to painful over-stimulation, thereby forcing the patient to manually adjust the output. There is a need for a method or system that would eliminate, or at least mitigate, the need to perform such manual adjustments. Such method or system would be of great benefit to the patient.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing a neural stimulation system that automatically corrects or adjusts the stimulus amplitude in order to maintain a comfortable and effective stimulation therapy. Auto correction of the stimulus amplitude is linked to the measurement of pressure changes.

Because the events that lead to the necessity of an output amplitude change are all associated with how much electrical energy is actually delivered to the neural tissue, and because how much energy is delivered to the neural tissue is, in part, a function of the proximity of the electrodes to the neural tissue, which in turn is a function of the relative pressure, the present invention uses a measure of the relative pressure as an indicator of the electrode's effectiveness in providing therapeutic stimulation. Thus, as an event occurs, such as a postural change, lead array movement, or scar tissue maturation, that allows more energy to couple from the emitting electrode or electrodes to the neural tissue, the more likely the stimulus will be painful, i.e., the more likely over-stimulation will occur. Conversely, as an event occurs that attenuates the coupled energy, the more likely the stimulus will not be sufficient to evoke the desired therapeutic effect (under stimulation).

As indicated above, the present invention uses the relative change in the measured pressure as an indicator of the quantity of current flow between the electrodes and the neural tissue. The measurement of the relative pressure changes of the neural tissues by the stimulation electrodes or neighboring electrodes provides a relative quantitative measure of the electrode array's theoretical effectiveness in providing therapeutic stimulation. The relative change in the measured pressure provides information on the proximity of the electrodes to the neural tissue. The measured relative pressure changes with respect to time allows can be used in the system as a signal to effectively auto correct the output amplitude, thereby minimizing the occurrence of over-stimulation or under-stimulation.

One application for the present invention is for a Spinal Cord Stimulation (SCS) system that automatically corrects or adjusts the stimulation amplitude in order to maintain a comfortable and effective paresthesia. As postural change, lead array movement, scar tissue maturation, and the like occur, allowing more energy to couple from the emitting electrode or electrodes of the SCS system to the neural tissue, the more likely over-stimulation occurs. Conversely, as events occur that attenuate the coupled energy, the more likely a desired paresthesia does not occur (under stimulation). The relative change in the measured pressure is used as a measure of stimulus energy delivered to the neural tissue. A correlation of the measured pressure changes may be obtained in the laboratory to predetermine the relationship between an increase in pressure and a corresponding increase or decrease in the subsequently applied stimulus energy. Alternatively, the morphology of the pressure wave resulting from an event, such as a postural movement may be correlated with the corrective adjustments to stimulus energy.

In accordance with one aspect of the invention, there is provided a neural stimulation system having means for measuring the relative pressure changes that occur over time in the epidural space where the electrodes are positioned so as to be in close proximity to the dura mater of the spinal cord. There can be an autocorrection means which takes the significant changes in relative pressure, i.e., changes indicating that over-stimulation or under-stimulation is likely to occur, and uses such pressure measurement as a trigger to automatically make corrective adjustments in the amplitude of the stimulus current to prevent painful over-stimulation or sub-threshold under-stimulation.

By way of example, as tissue healing (scar maturation) occurs following insertion of the electrode array, the measured relative pressure may decrease over time, requiring a higher stimulation pulse amplitude to achieve the same therapeutic result. The time frame is usually a slowly changing morphology of the relative pressure change, and a decrease in the measured relative pressure change with a corresponding increase in the amplitude of the output stimulus.

By way of another example, a rapidly changing relative pressure change indicates an acute movement of the electrode array with respect to the dura mater. Such acute movement could be due to a postural change causing the electrode array to move with respect to the location in the epidural space. Again, the changes in relative pressure and its morphology reflect the changing electrode environment, and the necessity to change the amplitude of the output stimulus. The present invention monitors the relative pressure change as a function of time and automatically makes adjustments in the amplitude of the output stimulus to maintain the desired therapeutic effect.

In another aspect of the invention an implantable neural stimulator comprises: an implantable pulse generator (IPG); a connector for connecting to at least one electrode; means for measuring relative pressure indicative of the coupling efficiency of an electrical stimulation current generated by the IPG; and means for automatically adjusting the energy of the stimulation delivered through the at least one electrode in order to compensate for variations in the measured coupling efficiency.

In yet another aspect of the invention, a method of neural stimulation for autocorrection of stimulation uses an implantable pulse generator (IPG). The IPG is connected to a lead having at least one electrode. In practice one electrode may be a stimulating electrode. However, in order to complete an electrical circuit, there is always a second return electrode present, whether it is another electrode or the IPG housing. The method of the present invention comprises: measuring relative pressure indicative of the coupling efficiency of an electrical stimulation current applied to neural tissue; correlating the decrease or increase in pressure measurement or, alternatively, the morphology of the pressure waveform with an increase or decrease in the stimulus energy supplied to the target neural tissue; and automatically adjusting the magnitude of subsequent stimulus energy delivered to the neural tissue through the at least one electrode attached to the lead in order to maintain an optimal level of therapy, wherein the adjustment of the stimulus magnitude is based on the relative measured pressure and its correlation to an increase or decrease in stimulus energy.

It is thus a feature of the present invention to provide a neural stimulation system wherein the output stimulus amplitude is automatically corrected or adjusted in order to compensate for coupling variations in the electrode-to-neural-tissue interface that cause more or less energy to reach the neural tissue from the electrode. In a preferred embodiment, such sensed coupling variations are determined by measuring or monitoring relative pressure changes that occur within the epidural space.

It is a further feature of the invention to provide a method of neural stimulation that includes measuring relative pressure changes near the location where a stimulus electrode array is located, wherein such pressure changes are indicative of the coupling efficiency of the electrical stimulation current to the neural tissue, and automatically adjusting the magnitude of subsequent stimulating current pulses in order to compensate for variations in the measured coupling efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 shows a representative neural stimulation system of the type with which the present invention may be used;

FIG. 2 shows the stimulation system of FIG. 1 being used as a Spinal Cord Stimulation (SCS) system, with the electrode array inserted alongside the spinal cord in the epidural space, in close proximity to the dura mater;

FIG. 3A is a block diagram of a system that automatically adjusts the amplitude of the stimulus current applied to neural tissue in accordance with the present invention;

FIG. 3B illustrates one method for generating and sensing relative pressure in accordance with the invention;

FIG. 4 is a block diagram of a representative Implantable Pulse Generator (IPG) that may be used to practice the present invention;

FIG. 5 is a timing waveform diagram that depicts various types of pressure measurements (P1, P1-D, P2, P3) that may be sensed as a function of time during operation of an SCS, or other neural stimulation, system; and

FIG. 6 is a high level flow chart that shows a method of practicing the invention in accordance with one embodiment thereof.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

A representative neural stimulation system 10 is shown in FIG. 1. Such system typically comprises an Implantable Pulse Generator (IPG) 12, a lead extension 14, an electrode lead 16, and an electrode array 18. The electrode array includes a plurality of electrode contacts 17 (also referred to as “electrodes”). The electrodes 17 are arranged, for example, in an in-line array 18 near the distal end of the lead 16. Other electrode array configurations may also be used. The IPG 12 generates stimulation current pulses that are applied to selected ones of the electrodes 17 within the electrode array 18. In accordance with the teachings of the present invention, a pressure transducer, or other pressure sensor, 17 b, may be carried on the body of the lead 16 near the distal end of the lead.

A proximal end of the lead extension 14 is removably connected to the IPG 12, and a distal end of the lead extension 14 is removably connected to a proximal end of the electrode lead 16. The electrode array 18, is formed on a distal end of the electrode lead 16. The in-series combination of the lead extension 14 and electrode lead 16, carry the stimulation current from the IPG 12 to electrodes of the electrode array 18. It should be noted that the lead extension 14 need not always be used with the neural stimulation system 10. The lead extension 14 is only needed when the physical distance between the IPG 12 and the electrode array 18 requires its use.

Turning next to FIG. 2, the neural stimulation system 10 is shown being used as a Spinal Cord Stimulator (SCS) system. In such configuration, the lead 16, and more particularly the electrode array 18, including a pressure sensor that may be included as part of the array, is implanted in the epidural space 20 of a patient so as to be in close proximity to the spinal cord 19. Due to the lack of space near the lead exit point 15 where the electrode lead 16 exits the spinal column, the IPG 12 is generally implanted in the abdomen or above the buttocks. The lead extension 14 facilitates locating the IPG 12 away from the lead exit point 15.

A more complete description of an SCS system may be found in U.S. patent application Ser. No. 09/626,010, filed Jul. 26, 2000, now issued as U.S. Pat. No. 6,516,227, which patent is assigned to the same assignee as is the present application, and is incorporated herein by reference in its entirety.

Next, with respect to FIG. 3A, there is shown a functional block diagram of a system that automatically adjusts the amplitude of the stimulus current applied to neural tissue in accordance with the present invention. As seen in FIG. 3A, an electrode 17 is placed in close proximity to neural tissue 24 that is to be stimulated. The electrode 17 is electrically connected to a current pulse generator 13 which generates a stimulus pulse having a magnitude that is set by magnitude adjust circuitry 115. The magnitude adjust circuitry 115 sets the magnitude of the stimulus pulse as specified by stimulation control circuitry 117. The stimulation control circuitry 117 usually comprises some sort of processor, or state logic, that operates in accordance with a stored program or state diagram. It initially sets the magnitude of the stimulus pulse to a programmed or predetermined value.

As the stimulus pulse is applied to the neural tissue 24, an appropriate pressure sensor S senses the coupling efficiency between the stimulus current and the neural tissue. That is, the pressure sensor S provides a measure of how effective the applied stimulus is at stimulating the neural tissue 24. The sensor S is connected to the magnitude adjust circuitry 115 so as to provide a feedback signal that indicates whether the magnitude of the stimulus needs to be adjusted up or down. For example, should the pressure measurement with sensor S indicate that very little of the energy is being coupled to the neural tissue 24, then the feedback signal provided through the sensor S automatically causes the magnitude adjust circuitry 115 to increase the magnitude of the stimulus pulse so that the total stimulus energy delivered to the neural tissue 24 remains about the same. Conversely, should the pressure measurement with sensor S indicate that more energy is being coupled to the neural tissue 24, then the feedback signal provided through the sensor S automatically causes the magnitude adjust circuitry 115 to decrease the magnitude of the stimulus pulse so that the total energy delivered to the neural tissue 24 remains about the same. Thus, it is seen that the magnitude adjust circuitry 115 automatically adjusts the magnitude, e.g., amplitude, of the stimulus pulse so that the energy coupled to the neural tissue remains more or less the same. It may be appreciated that while stimulus energy is generally determined by adjusting stimulus amplitude while holding pulsewidth and the frequency of the stimulus pulses (pulses per second) constant, it is also possible to vary the stimulus energy by changing one or more of the stimulus parameters: pulse amplitude, pulsewidth and frequency.

In accordance with the present invention, coupling efficiency is determined by measuring the relative pressure changes in the vicinity of the electrode-tissue interface. Such relative pressure measurement can provide a quantitative measure of the stimulating electrode's positional relationship relative to the target neural tissue, which determines the coupling efficiency between the electrode and neural tissue. For example, as the relative pressure increases, that means the electrode contact may have moved closer to the neural tissue 24, thereby making the coupling efficiency higher. Conversely, as the relative pressure decreases, that means the electrode contact may have moved farther away from the neural tissue 24, thereby making the coupling efficiency lower. Thus, for purposes of the present invention, the sensor S measures relative pressure.

Next, with respect to FIG. 3B, there is shown one method for measuring the relative pressure. Shown in FIG. 3B is a schematic representation of the distal end of the lead 16, including two electrode contacts 17 a and 17 c, and a suitable pressure transducer 17 b. Lead 16 used with the neural stimulation system will have at least one electrode contact, and will usually have a plurality of electrode contacts, e.g., four, eight, sixteen or more electrode contacts. The two electrode contacts 17 a and 17 c shown in FIG. 3B is intended to be illustrative only, and not limiting. It is also to be understood that the physical and electrical connection with the electrode contacts 17 a and 17 c, and the pressure sensor 17 b, is typically made through wires 19 that pass through the body of the lead 16. However, for simplicity of illustration in the schematic diagram of FIG. 3B, the electrical connections with the respective electrode contacts 17 a, 17 c and the pressure sensor 17 c, are shown by wires external to the body of the lead 16. The electrode contacts 17 a and 17 c, as well as the pressure sensor 17 b, are positioned in the epidural space near the dorsal column fibers 24 within or near the spinal cord.

In operation, a stimulation pulse, from a pulse generator 30, is applied to a selected pair of the electrode contacts, e.g., electrode contacts 17 a and 17 c. As connected in FIG. 3B, the polarity of the pulse generator 30 causes a current, represented by arrow 28, to be emitted from electrode contact 17 c to the neural tissue 24. The current 28 flows through the nerve tissue 24 and surrounding tissue and returns to the pulse generator 30 through electrode contact 17 a. The energy contained within the current 28 is coupled to the neural tissue 24 as a function of the coupling efficiency between electrode contacts 17 a, 17 c and the neural tissue 24. This coupling efficiency may vary, for many reasons, such as postural changes, relative movement between the lead 16 and tissue 24, or scar tissue maturation, to name just a few.

The electrode array 18 typically fits snugly within the epidural space next to the spinal column. There is thus a relative pressure applied against the electrode contacts 17 a and 17 c, as well as the pressure sensor 17 b. This relative pressure generally provides information on the proximity of the electrode contacts 17 a and 17 c to the neural tissue 24, and thus also provides a relative quantitative measure of the electrode array's theoretical effectiveness in providing therapeutic stimulation. Such relative pressure is schematically represented in FIG. 3B by the wavy lines 32.

The relative pressure 32 may be sensed in various ways. One way is through a pressure transducer 17 b. A typical pressure transducer is coupled to a voltage source 41 such that an electrical current flows through the transducer. As the relative pressure felt at the transducer changes, the impedance of the transducer changes, and thus the current that flows through the transducer changes. Hence, by monitoring changes in the current that flows through the transducer 17 b, a measure is provided of the relative change in pressure felt at the transducer. One way in which the current flowing through the transducer 17 b may be measured is by monitoring the voltage developed across a small resistance R connected in series with the transducer using a suitable amplifier 40, as shown in FIG. 3B. The voltage applied across the transducer 17 b may be a pulsed voltage or a constant (dc) voltage. The output signal obtained from the amplifier 40 thus becomes a measure of the sensed relative pressure.

It is to be emphasized that the technique shown in FIG. 3B for sensing relative pressure is only representative of various ways that may be used. For example, a suitable piezoelectric element, carried on the distal end of the lead body, may be biased with an appropriate voltage and coupled to an oscillator circuit wherein the frequency of oscillation is a function of pressure felt by the transducer. Additionally, any suitable “strain gauge” can be fashioned so as to be carried on the body of the lead 16, and therefore provide a means for measuring the relative pressure sensed at the electrode array, and hence provide a means for measuring the coupling efficiency between the electrode contacts and the neural tissue.

Further, it should also be noted that the pressure sensor or transducer need not be carried by or on the lead body. Rather, a separate, dedicated pressure transducer may be used to monitor the relative pressure in the vicinity of the electrode array. Such separate transducer may be included on a separate lead.

Turning next to FIG. 4, there is shown a functional block diagram of a representative Implantable Pulse Generator (IPG) 12 (or, with respect to FIG. 3, pulse generator 30) that may be used to practice the present invention. As seen in FIG. 4, the IPG 12 is connected to a multiplicity of electrode contacts E1, E2, . . . En, where n represents an integer of at least 2. The dotted-dashed line 102 in FIG. 4 represents the boundary between the outside of the IPG case (which is exposed to body tissues and fluids when the IPG is implanted) and the inside of the IPG case (which forms an hermetically sealed compartment wherein the electronic and other components are protected from the body tissues and fluids). Feed-through terminals 104 a, 104 b, . . . 104 n are thus used to provide an electrical path through the IPG case wall 102. The feed-through terminals 104 a, 104 b, . . . are electrically connected to the electrodes E1, E2, . . . En through wires within the lead 16.

Thus, it is seen that each electrode contact E1, E2, . . . En is connected through a respective feed-through terminal 104 a, 104 b, . . . 104 n to a respective circuit node 106 a, 106 b, . . . 106 n within the hermetically sealed IPG case. This node, in turn is connected to a P-DAC circuit 108 a and an N-DAC circuit 110 a. Each of the other circuit nodes 106 b, . . . 106 n within the IPG similarly have a respective P-DAC circuit and N-DAC circuit connected thereto.

A pressure transducer PX is also connected through feed through terminals 105 to a suitable measuring circuit. For example, a pulsed voltage source 113 is used to apply a voltage potential across the transducer PX, and then the current flowing through the transducer PX is measured, e.g., as the voltage developed across a series resistance R, using a suitable amplifier 112. Other techniques and monitoring circuitry, both digital and/or analog, may also be used to perform this measuring function, and a wide variety of different types of pressure transducers may likewise be used.

A case electrode, CASE, may also be provided that effectively provides a common or return electrode that may be used with some stimulation and sensing configurations, as required.

In operation, in order to generate a stimulus current pulse that is applied between electrodes E1 and E2, for example, the P-DAC circuit 108 a, as controlled by control logic 120 over data bus 122, causes a stimulation current having a specified amplitude to be emitted from the node 106 a, and hence to be emitted from the electrode contact E1. At the same time, the N-DAC circuit 110 b, similarly controlled by control logic 120, causes a stimulation current of the same magnitude to be received through node 106 b, and hence through electrode contact E2. (Not shown in FIG. 4, but assumed to be present, are coupling capacitors connecting the respective nodes 106 and feed-through terminals 104.) In this way, a precisely controlled current is generated that flows from electrode contact E1 to electrode contact E2 through whatever body and nerve tissue resides between electrodes E1 and E2. The duration of the current flow, i.e., the width of the current pulse that is generated, is controlled by timer logic circuitry 124. The operation of this output circuitry, including alternative embodiments of suitable output circuitry for performing the same function of generating current stimulus pulses of a prescribed amplitude and width, is described more fully in the above-referenced U.S. patent application Ser. No. 09/626,010, now issued as U.S. Pat. No. 6,516,227.

The relative pressure is monitored through the pressure transducer PX. Such monitoring may be on a continuous or sampled basis. If sampled, the sampling is preferably done at a time other than the time when the stimulation pulse is applied to the selected electrodes. The sense amplifier 112 connects to monitoring circuitry 126. One or more additional sense amplifiers 112 n may also monitor the voltage at various ones of the nodes 106 a, 106 b, 106 n, to thereby provide a means for measuring the voltage at selected nodes. Such voltage measurement in conjunction with the known current flowing between selected electrodes provides a means for determining the impedance between the selected electrodes. The impedance measurement, in turn, provides an alternative measure of the coupling efficiency between the electrodes selected to perform the stimulation. The monitoring circuitry 126 also monitors other signals 128 from various locations or components within the IPG, e.g., battery voltage, charge current, etc.

The control logic 120, the timer logic 124, and the monitoring circuit 126 are controlled or watched by a suitable micro-controller (μC) circuit 130. The μC circuit 130 is coupled to the control logic 120, the timer logic 124, and the monitoring circuitry 126 over data buses 132, 134 and 136, respectively.

Suitable memory circuitry 140 is likewise coupled to the μC 130, as is an oscillator and clock circuit 142. The μC 130, in combination with the memory circuit 140 and oscillator and clock circuit 142, thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory 140. Alternatively, for some applications, the function provided by the microprocessor system may be carried out by a suitable state machine.

Power for the IPG is provided by way of a suitable power source 144, such as a rechargeable or primary battery. A power circuit 146 can be used to control the charging or replenishment of the power source, as described more fully in the above-referenced patent application Ser. No. 09/626,010, now issued as U.S. Pat. No. 6,516,227.

The power circuit 146, the μC 130 and the monitoring circuitry 126 are also coupled to charging and telemetry circuitry 148. An antenna coil 150 is likewise coupled to the telemetry circuitry 148. It is through the antenna coil 150 that charging, forward telemetry and back telemetry signals may be received and sent to an external device, such as an external programmer or charging circuit, as described more fully in the above-referenced patent application Ser. No. 09/626,010. In practice, separate coils may be used for charging, forward telemetry and back telemetry functions, as described more fully in the above-referenced U.S. Pat. No. 6,516,227, but for purposes of the present invention those distinctions are not important.

In FIG. 4, the antenna coil(s) 150 is shown as being outside the hermetically sealed case of the IPG. In such configuration, feed-through terminals 103 are used to allow the coil(s) to be electrically connected to the charging and telemetry circuitry 148, which are inside the hermetically sealed case. Alternatively, if the case is made from a non-ferromagnetic material, such as titanium, or ceramic, the coil(s) 150 may be located inside of the hermetically sealed case.

It is to be emphasized that the schematic illustration of FIG. 4 is intended to be functional, and not limiting. Those of skill in the art will be able to fashion appropriate circuitry, whether embodied in digital circuits, analog circuits, software and/or firmware, or combinations thereof, in order to accomplish the desired functions.

FIG. 5 is a timing waveform diagram that depicts various types of representative pressure measurements (P1, P1-D, P2, P3) that may be sensed as a function of time. The measurements may be continuous or sampled. If sampled, a sampling pulse is generated at periodic intervals, and, e.g., the value of the pressure measurement P1 at the sample time is held as a fixed value until the next sample time, as represented by the waveform P1-D.

As seen in FIG. 5, at time t1, the three pressure measurements are all about the same value. At time t2, all three pressure measurements have increased slightly. The pressure measurement P1 continues to gradually increase through times t3, t4 and t5. Such gradual increase in the pressure measurement could be indicative of a slowly migrating lead.

In accordance with the present invention, and in response to sensing a slight increase in the relative pressure measurement P1 between times t1 through t5, the correction circuitry programmed or wired into the μC 130 can decrease the amplitude of the stimulation current by a small amount, thereby maintaining the stimulus energy delivered to the neural tissue at a predetermined, constant level or, alternatively, a predetermined level range. Had the relative pressure measurement P1 slightly decreased over the relevant time periods, then the amplitude of the stimulation current would have been increased a small amount.

As further seen in FIG. 5, shortly after time t2, the pressure measurement P2 drops quite rapidly and remains relatively low through times t3, t4 and t5. However, after time t5 the pressure returns to near its initial value. Such a sudden dip in the pressure measurement, with a return to its initial value, could be indicative of a temporary movement in the lead position caused, e.g., by a change in the patient's posture.

As additionally illustrated in FIG. 5, shortly after time t3, the pressure measurement P3 suddenly drops to a relatively low value, and remains low through times t4 and t5 and thereafter. Such a sudden permanent drop in the relative pressure measurement, with no return to near its initial value, could be indicative of a permanent movement or slippage of the lead.

In response to the changes in relative pressure measurements P2 or P3, correction circuitry programmed or wired into the μC 130 alters the amplitude (or energy) of the subsequently applied stimulus or stimuli to change by an amount aimed at maintaining the stimulus energy delivered to the neural tissue at a constant, efficacious level.

It is the energy content of the stimulus pulse that is adjusted in accordance with the invention when a change in the coupling efficiency has been detected. The energy content of the stimulus pulse is readily adjusted by adjusting the amplitude of the stimulus pulse. However, the energy content can also be adjusted by changing the width, or duration, of the stimulus pulse waveform, as well as the frequency with which the stimulus pulse is applied. Thus, as used herein, the term “amplitude”, or “magnitude”, although commonly used to signify a change in the value of a pulse, may be understood, instead, in the present context to broadly mean the “amplitude” or “magnitude” of the energy content of a stimulus pulse or a train of pulses. Thus, as used herein, a change of stimulus “amplitude” or “magnitude” of a pulse may be equivalently achieved by changing pulse amplitude, pulsewidth, frequency of the pulses or any combination of the three stimulus parameters thereof.

FIG. 6 is a high level flow chart that shows a method of practicing the invention in accordance with one embodiment thereof. As seen in FIG. 6, a first step involves programming the operating parameters (block 202) into the IPG circuitry. Such operating parameters include not only the operating regime for the neural stimulation system, e.g., stimulation pulse amplitude, pulsewidth, frequency, and electrodes, but also the parameters used by the invention to determine when a sufficient change in the sensed pressure has occurred to trigger the auto correction features of the invention.

As a preliminary matter, because various electrode types may be used in combination with different electrode configurations, it is preferred that each patient undergo a lab analysis to correlate the sensed pressure changes with a predicted adjustment to the magnitude of the adjusted stimulation in order to actually-deliver an approximately constant magnitude of stimulus energy at the actual target neural tissue. For example, the correlation provides an answer to whether an increase in relative pressure corresponds to an adjustment requiring an increase or decrease in the stimulus energy. To obtain this correlation data, the patient can be instructed to make various positional changes while a test stimulus, similar to that shown in FIG. 5, is applied through a stimulating electrode and responsive pressure measurements are recorded as a function of the positional changes. In addition, it may be necessary to vary the stimulus amplitude during the assessment test to determine the just noticeable stimulation (“perception threshold”) at a particular patient position and the “maximum comfortable” level of stimulation in order to interpolate and correlate the positional change with a change in stimulus energy delivered to the neural tissue. This correlation data can then be stored into memory 140 or other memory in the IPG which can then be recalled and used to analyze the indication of each sensed pressure measurement.

A more sophisticated method for correlating an event with a change in relative pressure employs the morphology of the pressure curve. A short-term event can produce a short-term relative pressure change having its own characteristic morphology such as shown by pressure waveforms P2 and P3 in FIG. 5. Pressure changes caused by different body movements may have different morphologies. If a pressure morphology can be linked to an associated corrective adjustment, when that same morphology is sensed by the IPG, the proper corrective adjustment may be applied automatically.

A specific method of implementing this linkage is to generate a correlation table (“look-up” table) which may be developed, for example, for different body movements. A short-term postural change (“an event”) may occur over a time, T_(e). The types of postural events that should be included in the catalog of short term body movements are those movements normally made during the day, e.g., sitting to lying, standing to lying, lying to standing, sitting to standing and vice versa, twisting the torso to one or the other side, running, etc. Each of these events must be characterized in the laboratory for each patient to generate a personalized look-up table that correlates the body movements/events with a characteristic pressure morphology, over a time, T_(e). In addition, for each event characterized, there is linked a corresponding predicted adjustments (up or down) to the stimulus energy, which may be adjusted, through magnitude adjust 115, by varying pulse amplitude, pulsewidth, and pulse frequency. To obtain usable pressure morphology associated with each body movement, it is necessary that a sufficient number of pressure values are sampled with the event time, T_(e). The IPG 12 can have automatic capability to capture pressure datapoints over a total recording time, T_(r), which should be greater than T_(e). These data may be uplinked to an external programmer (not shown) having processing capability or, alternatively, a computer which can be used to analyze and set the corrective stimulus adjustments. After the entire look-up table is generated, including corrective stimulus adjustments linked one-to-one to each stored characteristic pressure waveform morphology, the data table can be downloaded to the IPG to be stored in memory. In accordance with the method of the present invention, this look-up table may then be recalled by software in the IPG to make nearly instantaneous corrective adjustments to stimulus energy as a function of an identified short-term, pressure waveform morphology.

Once all the operating parameters have been programmed, a determination is made as to whether the auto correction feature of the invention has been programmed ON (block 204). If not, then the stimulator operates in accordance with its programmed operating regime without invoking the auto correction feature (block 206). Unless such programmed operating regime is turned off (block 208), this process continues.

If auto correction is turned ON (block 204), then a pressure measurement is made (block 214), either by pulsing the pressure transducer or sampling the transducer output. Once the relative pressure has been measured, then a determination is made as to whether the relative pressure has changed (block 216). If not, then the operation of the stimulator continues per its programmed parameters (block 206), and the process continues.

If a determination is made that the relative pressure measurement has changed (block 216), then the operating parameters, e.g., stimulation current amplitude, are adjusted to compensate for the change in energy coupling (block 218), and the stimulator is operated in accordance with such newly adjusted parameters (block 206). The selection of the stimulus amplitude (or energy) may be based on predetermined calculations or based on a pre-programmed lookup table as previously described. In this manner, negative feedback is provided through use of the pressure measurement to maintain the energy coupling and the energy delivered to the neural tissue at substantially a constant (or programmed) level. The set level for maintaining the energy delivered to the neural tissue may be a pre-programmed single level or it may be a pre-programmed range that is above the just noticeable stimulation (“perception threshold”) and below the maximum comfortable stimulation. Stimulation energies above the maximum comfortable stimulation level will cause pain.

It is emphasized that while the present invention has been explained specifically in the context of the SCS application, the present invention can be practiced where the nerve stimulated is a peripheral as well as a spinal cord nerve. All figures except FIG. 2 apply equally to the case of stimulating peripheral nerve. FIG. 1 shows an in-line electrode array 18. In comparison, in peripheral nerve application the lead type and electrode configuration used may be different. Peripheral nerve application also differs from SCS application in that the stimulation provides different physical results. In SCS, sensory fibers are stimulated. In peripheral nerve application, motor nerves are stimulated which contract voluntary muscle such as in the arms and legs or involuntary, smooth muscle, such as those for constricting veins. The method and system of the present invention is not dependent, however, on a specific type of lead or electrode configuration used. Rather, the present invention can be applied equally effectively for the purpose of autocorrecting stimulus energy applied to peripheral nerve stimulation following a detected change in relative pressure. Thus, it is within the contemplation of the present invention to include within its scope the application where peripheral nerve is stimulated.

As described above, it is thus seen that the present invention provides a neural stimulation system wherein the output stimulus magnitude is automatically corrected or adjusted in order to compensate for coupling efficiency variations in the electrode-to-neural-tissue interface that cause more or less energy to reach the neural tissue from the electrode. Variations in the coupling efficiency are determined, in a preferred embodiment, by sensing relative changes in the measured pressure in the vicinity of the electrode-tissue interface.

As further described above, it is seen that the invention provides a method of neural stimulation that includes measuring relative pressure at or near the electrode-tissue interface, which measurements are indicative of the coupling efficiency of the electrical stimulation current to the neural tissue, and then automatically adjusting the magnitude of subsequent stimulus pulses in order to compensate for variations in the measured coupling efficiency.

While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. 

1. A method of neural stimulation for autocorrection of stimulation using an implantable pulse generator (IPG) that is connected to a lead having at least one electrode, the method comprising: measuring relative pressure within the epidural space indicative of the coupling efficiency of an electrical stimulus applied to the neural tissue; correlating a decrease or increase in pressure measurement with an increase or decrease in stimulation energy delivered to the neural tissue; and automatically adjusting the stimulation energy delivered to the neural tissue through the at least one electrode connected to the lead in order to maintain an optimal level of therapy based on the correlation determination in the previous step.
 2. The method of claim 1 wherein measuring the relative pressure comprises: placing a pressure sensor in the vicinity of an electrode-tissue interface; monitoring the pressure sensor over time to provide a pressure measurement; and using changes in the relative pressure measurement as a feedback signal to automatically adjust the magnitude of subsequent stimulation energy.
 3. The method of claim 2 wherein the pressure sensor is mounted to the electrode lead.
 4. The method of claim 2 wherein the pressure sensor is mounted to a second lead.
 5. The method of claim 2 wherein monitoring the pressure sensor comprises sampling the pressure sensor at specified sampling times.
 6. The method of claim 2 wherein the step of automatically adjusting the stimulation energy delivered to the neural tissue through the at least one electrode connected to a lead comprises adjusting pulse amplitude, pulsewidth, frequency of pulses, or a combination thereof.
 7. The method of claim 1 wherein the optimal level of therapy is a predetermined therapeutic range of stimulation energy actually delivered to the neural tissue that elicits a response that is below that which causes a sensation of pain or discomfort, which is the maximum comfortable threshold.
 8. The method of claim 1 wherein correlating a decrease or increase of the pressure measurement with an increase or decrease in the stimulation energy supplied to the neural tissue comprises: having a patient implanted with the IPG make postural movements to induce changes in measured pressure; and recording an associated decrease or increase in pressure as well as sensation; providing a predictive stimulation energy to be applied as a corrective adjustment; saving the pressure waveform and linked stimulation energy data into an IPG memory for recall and use in the step of automatically adjusting the stimulation energy delivered to the neural tissue.
 9. The method of claim 1 wherein the step of automatically adjusting the stimulation energy delivered to the neural tissue through at least one electrode attached to the lead comprises adjusting pulse amplitude, pulsewidth, frequency of pulses or a combination thereof.
 10. A neural stimulation system for autocorrection of stimulation, the system comprising: at least one electrode mounted on a first lead; an implantable pulse generator connected to the first lead, wherein the pulse generator provides stimulation current pulses to the at least one electrode at a prescribed magnitude; means for measuring pressure in the vicinity of an electrode-tissue interface within the epidural space; means for determining a change in measured pressure over time; and auto correction means for adjusting the stimulation energy as a function of the change detected in pressure measurements over time.
 11. The neural stimulation system of claim 10 wherein the means for measuring pressure comprises a pressure transducer mounted on the same first lead having the at least one electrode.
 12. The neural stimulation system of claim 10 wherein the means for measuring pressure comprises a pressure transducer that is located on a second lead.
 13. The neural stimulation system of claim 10 wherein the means for measuring pressure comprises a strain gage.
 14. The neural stimulation system of claim 10 wherein the means for measuring pressure comprises a piezo electric element.
 15. An implantable neural stimulator comprising: an implantable pulse generator (IPG); a connector for connecting to a lead having at least one electrode; means for measuring relative pressure within the epidural space indicative of the coupling efficiency of an electrical stimulation current generated by the IPG; and means for automatically adjusting the stimulation energy of the stimulation delivered through the at least one electrode in order to compensate for variations in the measured coupling efficiency.
 16. The implantable neural stimulator of claim 15 further comprising memory that stores data in the IPG, which data correlate changes in pressure measurement with adjustments to the stimulation energy to maintain optimal therapy.
 17. The implantable neural stimulator of claim 15 wherein the means for automatically adjusting the stimulation energy delivered through the at least one electrode comprises a magnitude adjust device incorporated in the IPG for adjusting pulse amplitude, pulsewidth, frequency of pulses or a combination thereof.
 18. A method of neural stimulation for autocorrection of stimulation using an implantable pulse generator (IPG) that is connected to a lead having at least one electrode, the method comprising: measuring relative pressure within the epidural space indicative of the coupling efficiency of an electrical stimulation current applied to neural tissue; and automatically adjusting the magnitude of subsequent stimulation energy delivered to the neural tissue through the at least one electrode connected to the lead in order to maintain an optimal level of therapy, wherein the adjustment of the stimulation energy is based on the relative measured pressure and its correlation to an increase or decrease in stimulation energy.
 19. The method of claim 18 wherein measuring relative pressure comprises: placing a pressure sensor in the vicinity of an electrode-tissue interface; monitoring the pressure sensor over time to provide a pressure measurement; and using changes in relative pressure measurement as a feedback signal to adjust the magnitude of subsequent stimulation energy.
 20. The method of claim 19 wherein the pressure sensor is mounted to the electrode lead.
 21. The method of claim 18 wherein the pressure sensor is mounted to a second lead.
 22. The method of claim 19 wherein monitoring the pressure sensor comprises sampling the pressure sensor at specified sampling times.
 23. The method of claim 19 wherein the step of automatically adjusting the magnitude of subsequent stimulation energy delivered to neural tissue through the at least one electrode attached to a lead comprises adjusting pulse amplitude, pulsewidth, frequency of pulses, or a combination thereof.
 24. The method of claim 18 wherein the optimal level of therapy is a predetermined therapeutic range of stimulation energy actually delivered to the neural tissue that elicits a response that is below that which causes a sensation of pain or discomfort.
 25. The method of claim 18 wherein the step of automatically adjusting the magnitude of subsequent stimulation energy delivered to the neural tissue through the at least one electrode connected to the lead comprises adjusting pulse amplitude, pulsewidth, frequency of pulses or a combination thereof.
 26. The method of claim 18 further comprising: linking each characteristic morphology caused by an event to a particular corrective adjustment for delivering a predetermined stimulation energy to the neural tissue; storing the linked information between each characteristic morphology to a particular corrective adjustment in the IPG memory; and recalling the linked information in the step of adjusting the magnitude of subsequent stimulation energy.
 27. The method of claim 26, wherein the event is a postural movement; and wherein the step of linking each characteristic morphology caused by an event to a particular corrective adjustment comprises measuring a pressure waveform for each postural movement while a patient with an IPG implanted makes various postural movements. 